From time immemorial toy dolls have been a source of joy and comfort to children who use them to role play child-adult relationships and to explore parental role models. The doll will often acquire a personality in the child's mind and will be used to play-act various behaviour patterns with which the child is familiar or which the child is itself learning to accommodate. Doll manufacturers are well aware of the importance of simulation and try to assist the child's imagination by producing dolls which are as life-like as possible. Thus many dolls are now produced with eyes that close when the doll is placed in a recumbent position to simulate sleep. More recently, doll manufacturers have produced dolls which simulate ingestion of food and drink, examples of which are described in U.S. Pat. Nos. 3,855,729; 3,858,352; 4,504,241; and, 5,083,962.
When assessing the commercial potential of a toy one of the factors taken into consideration is the so-called "play value" of the toy. The play value is to some extent a measure of the capacity of the toy to hold a child's attention and to evoke interest and involvement. The play value of a toy doll may be enhanced by increasing the number of moving parts, improving the doll's life-like appearance and/or by incorporating various simulated physiological functions as noted above. One of the difficulties faced by doll manufacturers is that such simulated physiological functions are by nature very mechanical and therefore predictable--if the child presses this button or operates that lever then the doll will do the same thing every time. This predicability can in fact reduce the long term play of the doll as it leaves little room for imagination and the child quickly becomes bored.